Coherent Control Of Localized Surface Plasmon Excited By Femtosecond Laser

Surface plasmons are waves that propagate along the surface of a conductor,due to the collective oscillations of metal's conduction electrons that are induced by resonantly driving of the incident field.Illumination of a metallic nanoparticle which is much smaller than the wavelength of the incident light results in non-propagating excitations of the conduction electrons of metallic nanostructures that are called localized surface plasmons(LSPs).LSPs,due to the characteristics of confinement of electromagnetic fields beyond diffraction limit,are widely used in sensing,detecting and biomedicine.Especially merging LSPs and ultrafast laser technics results in femtosecond localized surface plasmons(FLSPs)which owns the characters of femtosecond time scale and ultra-strong peak intensity of electric field,it has attracted special interests due to applications in ultrafast modulator,ultrafast electron source and UV generation.The premise of the FLSPs being widely used in different areas is that we need to be able to effectively control its excitation position and intensity.The active control of the location of the excitation point supports and guarantees further development of these studiese.Therefore,effective control of the FLSPs is important for the aforementioned applications and development of the field.In this thesis,the physical mechanism and properties of coherent control of FLSPs in plasmonic nanostructures are studied in detail.The active coherence control of FLSPs in nanocross,bowtie antenna and nanoring was realized by various control methods.Finite difference time domain(FDTD)method was used to study the coherent control of FLSPs in nanostructures.Direct imaging of coherent control of FLSPs in nanostructures was also carried out by Ultrafast Photoemission electron microscopy(UF-PEEM).Initially,FDTD simulation software(FDTD solutions)was used to simulate the optical response of FLSPs in asymmetric nanocross structure.The results show that we can achieve the coherent control of FLSPs in the apices of four arms in the asymmetric nanocross structure by changing the wavelength.Moreover,the mechanism of this asymmetric nanocross structure excitation further demonstrated that the vertical arms serve as a stack of resonant parallel arms to support the resonance of parallel arm,therefore coherent control can be achieved simultaneously in the apices of four arms under the conditions of only changing the wavelength.Moreover,coherent control of FLSPs in the bowtie structure under resonance and off-resonant excitation was studied.FDTD simulation software was used to simulate the optical response of FLSPs in bowtie structure under two different control methods.Coherent control of FLSPs was carried out by changing the polarization of single femtosecond beam and the relative phase delay of vertically polarized double femtosecond beams.The results show that the excitation position locates in the tips of bowtie under resonant condation and in the edges of bowtie under off-resonant condition.Coherent control of FLSPs in bowtie nanostructure under both of resonant or off-resonant excitation can be achieved by changing the polarization of single beam and the relative phase delay of vertically polarized double beams.In addition,since the off-resonant FLSPs are significantly affected by the random defects excitation,the visualization of the PEEM image under off-resonant excitation is significantly lower than the case under resonant excitation.Through the linear superposition theory,we also analyzed the coherence control of FLSPs in the bowtie nanostructure under different excitation methods.The results show that control of the position or the phase of the excitation points can both be realized by these two control methods.Furthermore,coherent control of high order FLSPs in nanoring structures was studied.Polarization and wavelength modification of laser was used to control the high order FLSPs in the nanoring structure with directly imaging with PEEM.The results show that coherent control of the high order FLSPs in the nanoring structure can be achieved by the method of polarization and wavelength modification.However,due to the low near-field intensity of the FLSPs supported by the nanoring structure,moreover no cutting-edges in nanoring itself,the PEEM image of high order FLSPs in the nanoring structure is strongly affected by random defects excitation.Finally,in order to solve the problem of the effects of random defective excitation upon the PEEM images,a method of two-color femtosecond laser excitation of FLSPs that can effectly improve the clarity of PEEM image was proposed.The optimal characterization of PEEM image of the FLSPs under two-color femtosecond laser excitation was studied theoretically and experimentally.The results show that by using of two-color excitation method,we can effectively reduce the nonlinear order of the photoemission electrons related with the FLSPs and the effects of random defects excitation to the PEEM images,which leads us to obtain a more clear near field PEEM image of the FLSPs in nanostructures.